PHYSICS · Multi-part · Fermi University Physics · Earth, Climate & Energy Physics · Seismology, tectonics and geodynamics · Seismology, tectonics and geodynamics
The Solid Earth, Ch Stress, strain and elastic waves in the Earth
Problem
In an active continental rift, newly formed normal faults mapped from seismic-reflection and outcrop data dip at about 55∘ from the horizontal.
((a)) Using Anderson's theory of faulting, state which principal stress must be vertical for such faults to form, and use the Coulomb dip relation to find the implied angle of internal friction φf and coefficient of friction μf of these faults.
((b)) Laboratory triaxial tests on intact rock typically give μf≈0.6. Compare this with your answer and suggest, in words, one physical reason a mapped population of natural fault dips might imply a lower apparent friction than the laboratory value. (Idealisation: the dip relation used here strictly applies to faults forming for the first time in intact rock under the present stress state; it need not apply to reactivated, pre-existing weak faults, and mapped dips may include faults that have rotated since they formed.)